Using the mouse as a model for fetal development, we are using magnetic resonance microscopy to characterize the development of the cardiovascular system in normal and genetically altered mouse embryos. The mice are removed from the womb, injected through the umbilical vein with a contrast agent, and imaged. A variety of different genetic defects will be studied in embryos at a variety of stages in order to delineate clearly the developmental process and understand the nature of the defects. The great advantage of MRM is that it is quantifiable, and a comparison of the anatomy between normal and mutant mouse embryos at different stages will provide important clues as to the precise cause of fetal demise in the mutants. This knowledge will act as an important guide for future work in vivo in mice, and act as a first step towards the goal of developing a program of fetal imaging in humans. We also plan to use the system to characterize blood pool contrast agents which are being developed specifically for this work and for use in humans.